Process for secondary recovery

ABSTRACT

Hydrocarbons are recovered from a subterranean hydrocarbon-bearing formation penetrated by an injection well and a production well by displacing hydrocarbons toward the production well using a drive fluid such as water thickened with polyacrylamide or partially hydrolyzed polyacrylamide alkoxylated with an alkylene oxide and, if desired, a small amount of a sulfonated, alkoxylated polyacrylamide, i.e., an N-substituted polyacrylamide having N-substituents of the formula: 
     
         --(CH.sub.2 CH.sub.2 O).sub.m-1 CH.sub.2 CH.sub.2 SO.sub.3 M, 
    
     where m is an integer of from 2 to 5 and M is selected from the group consisting of hydrogen, sodium, potassium or ammonium. Optionally, the drive fluid can be saturated with carbon dioxide, nitrogen and/or natural gas at the injection pressure.

This application is a Continuation-In-Part of application Ser. No.06/245,875, filed Mar. 23, 1981, now abandoned, which is in turn aDivisional of application Ser. No. 06/122,323, filed Feb. 19, 1980,abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well wherein a drive fluid such as water thickenedwith polyacrylamide or partially hydrolyzed polyacrylamide or thesodium, potassium or ammonium salt thereof alkoxylated with ethyleneoxide or a mixture of ethylene oxide and propylene oxide and/or a smallamount of sulfonated, ethoxylated polyacrylamide bearing N-substituentsof the formula:

    --(CH.sub.2 CH.sub.2 O).sub.m-1 CH.sub.2 CH.sub.2 SO.sub.3 M,

is utilized to displace hydrocarbons in the formation toward aproduction well.

2. Prior Art

The production of petroleum products is usually accomplished by drillinginto a hydrocarbon-bearing formation and utilizing one of the well-knownrecovery methods for the recovery of hydrocarbons. However, it isrecognized that these primary recovery techniques may recover only aminor portion of the petroleum products present in the formationparticularly when applied to reservoirs of viscous crudes. Even the useof improved recovery practices involving heating, miscible flooding,water flooding and steam processing may still leave up to 70-80 percentof the original hydrocarbons in place.

Thus, many large reserves of petroleum fluids from which only smallrecoveries have been realized by present commercial recovery methods,are yet to reach a potential recovery approaching their estimatedoil-in-place.

Water flooding is one of the more widely practiced secondary recoverymethods. A successful water flood may result in recovery of 30-50percent of the original hydrocarbons left in place. However, generallythe application of water flooding to many crudes results in much lowerrecoveries.

The newer development in recovery methods for heavy crudes is the use ofsteam injection which has been applied in several modifications,including the "push-pull" technique and through-put methods, and hasresulted in significant recoveries in some areas. Crude recovery of thisprocess is enhanced through the beneficial effects of the drasticviscosity reduction that accompanies an increase in temperature. Thisreduction in viscosity facilitates the production of hydrocarbons sinceit improves their mobility, i.e., it increases their ability to flow.

However, the application of these secondary recovery techniques todepleted formations may leave major quantities of oil-in-place, sincethe crude is tightly bound to the sand particles of the formation, thatis, the sorptive capacity of the sand for the crude is great. Inaddition, interfacial tension between the immiscible phases results inentrapping crude in the pores, thereby reducing recovery. Anotherdisadvantage is the tendency of the aqueous drive fluid to finger, sinceits viscosity is considerably less than that of the crude, therebyreducing the efficiency of the processes. Another disadvantage is thetendency of the aqueous drive fluid to remove additional gas bydiffusion from the in-place oil thus further reducing the alreadylowered formation oil volume and increasing the viscosity of the oil.

As evidenced by U.S. Pat. Nos. 2,827,964; 2,842,492; 3,039,529;3,282,337; 3,476,186 and others, the art has found that channelingproblems may be lessened by the utilization of displacing media,generally consisting of an aqueous liquid which contains a water solubleagent to increase viscosity. For example, in U.S. Pat. No. 3,804,173(Jennings) polyacrylamide derivatives of the formula: ##STR1## aredisclosed as being valuable water soluble viscosity increasing agents.

There is a definite need in the art for a water flooding process inwhich the disadvantages discussed above are largely eliminated oravoided.

SUMMARY OF THE INVENTION

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well which comprises:

(A) injecting into the formation via an injection well a drive fluidcomprising water having dissolved therein a small amount of (1)polyacrylamide or (2) partially hydrolyzed polyacrylamide or the sodium,potassium or ammonium salt thereof alkoxylated with ethylene oxide or amixture of ethylene oxide and propylene oxide wherein the weight percentof ethylene oxide in the said mixture is from about 60 to about 95, andabout 0.01 to about 5.0 weight percent of a sulfonated, ethoxylatedpolyacrylamide or the sodium, potassium or ammonium salt thereof,

(B) forcing the said fluid through the formation and

(C) recovering hydrocarbons through the production well.

Applicant's invention is distinguished from prior art in that itemploys, among other things, a sulfonated, ethoxylated polyacrylamide orthe sodium, potassium or ammonium salt thereof as a drive fluidthickening agent. The ether linkage present in such polymers is believedto increase the efficiency of said viscosity increasing agents overthose previously disclosed in the art.

In another embodiment of this invention the drive fluid may comprisewater or brine having dissolved therein a small amount, i.e., about 0.01to about 5.0 weight percent of a sulfonated, ethoxylated polyacrylamideor the sodium, potassium or ammonium salt thereof.

An additional embodiment of this invention relates to the driving fluidcompositions utilized in step (A) which may be saturated with carbondioxide, nitrogen, natural gas and mixtures thereof, if desired.

DETAILED DESCRIPTION OF THE INVENTION

Prior to practicing the process of this invention it is sometimesdesirable to open up a communication path through the formation by ahydraulic fracturing operation. Hydraulic fracturing is a well-knowntechnique for establishing a communication path between an injectionwell and a production well. Fracturing is usually accomplished byforcing a liquid such as water, oil or any other suitable hydrocarbonfraction into the formation at pressures of from about 300 to about 3000psig which are sufficient to rupture the formation and to open upchannels therein. By use of this method it is possible to position thefracture at any desired vertical location with respect to the bottom ofthe oil-filled zone. It is not essential that the fracture planes behorizontally oriented, although it is, of course, preferable that theybe. After the fracture has been established, and without diminishing thefracture pressure, a propping agent may be injected into the fracture inorder to prevent healing of the fracture which woul destroy itsusefulness for fluid flow communication purposes. Gravel, metal shot,glass beads, sand, etc. and mixtures thereof are generally employed aspropping agents. When sand is utilized as the propping agent particleshaving a Tyler mesh size of from about 8 to about 40 are preferred(i.e., from about 0.016 to about 0.093 inches).

Generally, the number average molecular weight of the polyacrylamide orthe partially hydrolyzed polyacrylamide or the sodium, potassium orammonium salt thereof utilized in preparing the alkoxylated products ofthis invention will be such that the number average molecular weight ofthe alkoxylated polymers will range from about 10,000 to about 8,000,000or more. Such polyacrylamide materials are manufactured and sold by anumber of chemical manufacturers and are prepared by the usual vinylcompound polymerization methods.

The partially hydrolyzed polyacrylamides which are useful in preparingthe ethoxylated, partially hydrolyzed polyacrylamides employed in thedrive fluid of this invention consist of about 12 to about 67 molepercent of recurring units of the formula: ##STR2## where M representshydrogen, sodium, potassium or ammonium and about 33 to 88 mole percentof recurring units of the formula: ##STR3## The preparation of suchpartially hydrolyzed polyacrylamides is well known in the art and isdescribed in detail in U.S. Pat. Nos. 3,039,529; 3,002,960; 3,804,173,etc.

The alkoxylated polymers employed in the process of this inventioncomprise (1) polyacrylamide or (2) partially hydrolyzed polyacrylamideor the sodium, potassium or ammonium salt thereof alkoxylated with,i.e., reacted with, from about 2 to about 150 percent by weight ofethylene oxide or with a mixture of ethylene oxide and propylene oxidewherein the weight percent of ethylene oxide in the said mixture isabout 60 to about 95. In another embodiment, alkoxylated polymers usefulin the secondary recovery process of this invention are prepared byreacting polyacrylamide or partially hydrolyzed polyacrylamide with2,3-epoxy-1-propanol (i.e., glycidol).

The alkoxylation of the acrylamide polymers, i.e., the reaction of thesepolymers with an alkylene oxide, can be conveniently conducted usingmethods well known in the art. For example, an aqueous solutioncomprising about 10 to about 30 weight percent or more of the acrylamidepolymer in water along with about 0.5 weight percent or more of powderedpotassium hydroxide or sodium hydroxide is charged to an autoclave andthe autoclave and contents heated to a temperature of about 125° toabout 200° C. after which the required weight of ethylene oxide or amixture of ethylene oxide and propylene oxide is pressured with nitrogeninto the autoclave over a period of 1 to about 3 hours or more followingwhich the autoclave is allowed to cool to room temperature and thenvented. The reaction product remaining after being stripped to removevolatile materials yields the water-soluble, alkoxylated polymer.

A number of other methods are set out in the art for conducting suchalkoxylation reactions including those described in U.S. Pat. Nos.2,213,477; 2,233,381; 2,131,142; 2,808,397; 3,879,475; 2,174,761;2,425,845; 3,062,747; 3,380,765 and German Offen. 2,021,066 of Nov. 11,1971 (CA 76 86780b).

The following example which illustrates the preparation of thealkoxylated acrylamide polymers is to be considered not limitative.

EXAMPLE I

A total of 450 cc of water, 5 g. of powdered potassium hydroxide and 65g. of polyacrylamide (number average molecular weight of about 250,000)are added to an autoclave which is then heated to a temperature of 125°C. Ethylene oxide in the amount of 40 g is added to the autoclave undernitrogen pressure over a 1.05 hour period during which time thetemperature of the autoclave is maintained at 130° C. Next, theautoclave and contents are allowed to cool to room temperature afterwhich the autoclave is vented. The reaction mixture is then stripped ofvolatiles using a nitrogen purge. The resulting water-soluble product ispolyacrylamide alkoxylated with about 37 weight percent of ethyleneoxide.

In the secondary recovery process of this invention, generally theaqueous drive fluid will contain from about 0.01 to about 5.0 weightpercent or more of alkoxylated polyacrylamide or alkoxylated partiallyhydrolyzed polyacrylamide or the sodium, potassium or ammonium saltthereof and, optionally, about 0.01 to about 5.0 weight percent of thewater-soluble, sulfated, ethoxylated polyacrylamide or the sodium,potassium or ammonium salt thereof. Optionally, the aqueous drive fluidmay be saturated with carbon dioxide, nitrogen and/or natural gas at theinjection pressure which generally will be from about 300 to about 3000psig or more.

When polyacrylamide is reacted with an alkylene oxide where the alkylenegroup has from 2 to 4 inclusive carbon atoms and in the presence ofsodium or potassium hydroxide the reaction is one which takes place withthe active hydrogen atoms attached to the nitrogen atom of the --NH₂group resulting in the formation of alkoxylated polyacrylamide, i.e., anN-substituted, water-soluble polymer bearing N-substituents having theformula:

    --(RO).sub.n H

wherein R is alkylene or hydroxyalkylene of from 2 to 4 inclusive carbonatoms and n is an integer of at least 1.

The water-soluble, sulfonated, ethoxylated polyacrylamides useful in thedrive fluids of this invention are N-substituted polyacryamides havingnumber average molecular weights of about 10,000 to about 8,000,000 ormore and bearing N-substituents having the formula:

    --(CH.sub.2 CH.sub.2 O).sub.m-1 CH.sub.2 CH.sub.2 SO.sub.3 M,

wherein m is an integer of from 2 to 5 and M is selected from the groupconsisting of hydrogen, sodium, potassium or ammonium. Generally in thewater-soluble, sulfonated, ethoxylated polyacrylamides, the weightpercent of the N-substituents of the formula:

    --(CH.sub.2 CH.sub.2 O).sub.M-1 CH.sub.2 CH.sub.2 SO.sub.3 M,

where m and M have the same meaning as previously described will rangefrom about 5 to about 65 or more. These water-soluble, sulfonated,ethoxylated polymers of this invention can be prepared according toknown methods by first reacting ethoxylated polyacrylamide, i.e.,polyacrylamide bearing N-substituents of the formula:

    --(CH.sub.2 CH.sub.2 O).sub.m H,

wherein m is an integer of from 2 to 5, and having a number averagemolecular weight of about 10,000 to about 8,000,000 or more withsulfuric acid or any other suitable sulfating agent to form a sulfated,ethoxylated polyacrylamide bearing N-substituents of the formula:

    --(CH.sub.2 CH.sub.2 O).sub.m SO.sub.3 H,

wherein m is an integer of from 2 to 5. The above-described sulfated,ethoxylated polyacrylamides can be converted to the sulfonate byprocedures known in the art such as by reaction with sodium sulfite atan elevated temperature, i.e., about 150° to 185° C. for about 5 to 10hours or more. The corresponding sodium, potassium or ammoniumderivative of these water-soluble, sulfonated, ethoxylatedpolyacrylamides can be formed by neutralization with the correspondingalkali metal hydroxide or ammonium hydroxide. The preparation of thestarting polymers, i.e., the ethoxylated polyacrylamides, is decribed indetail in U.S. Pat. No. 2,808,397 which is incorporated herein in itsentirety by reference.

If desired, the aqueous drive fluids having dissolved therein theabove-described polymeric thickening agents may be made alkaline byaddition of an alkaline agent. The advantageous results achieved withthe aqueous alkaline medium used in the process of this invention arebelieved to be derived from the wetability improving characteristics ofthe alkaline agent.

Useful alkaline agents include compounds selected from the groupconsisting of alkali metal hydroxides, alkaline earth metal hydroxides,and the basic salts of the alkali metal or alkaline earth metals whichare capable of hydrolyzing in an aqueous medium to give an alkalinesolution. The concentration of the alkaline agent employed in the drivefluid is generally from about 0.005 to about 0.3 weight percent. Also,alkaline materials such as sodium hypochlorite are highly effective asalkaline agents. Examples of these especially useful alkaline agentsinclude sodium hyroxide, potassium hydroxide, lithium hydroxide,ammonium hydroxide, sodium hypochlorite, potassium hypochlorite, sodiumcarbonate and potassium carbonate.

A wide variety of surfactants such as linear alkylaryl sulfonates, alkylpolyethoxylated sulfates, etc. may also be included as a part of anyaqueous drive fluid compositions previously described. Generally about0.001 to about 1.0 or more weight percent of the surfactant will beincluded in the drive fluid.

This invention is best understood by reference to the following exampleswhich are offered only as an illustrative embodiment of this inventionand are not intended to be limitative.

EXAMPLE II

In a field in which the primary production has already been exhausted,an injection well is completed in the hydrocarbon-bearing formation andperforations are formed between the interval of 7025-7040 feet. Aproduction well is drilled approximately 415 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 7030-7045 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

In the next step water saturated with nitrogen at 1125 psig at atemperature of about 70° F. to which there has been added about 0.23weight percent sodium hydroxide and containing dissolved therein 0.35weight percent of N-substituted polyacrylamide bearing N-substituents ofthe formula:

    --(CH.sub.2 CH.sub.2 O)CH.sub.1 CH.sub.2 SO.sub.3 H, having a number average molecular weight of about 3,100,000 alkoxylated with about 14 weight percent of ethylene oxide in the manner previously described in Example I above is injected via the injection well into the formation at a pressure of about 1125 psig and at the rate of 0.90 barrel per minute. Injection of the driving fluid is continued at the rate of about 1 barrel per minute and at the end of 68 days the rate of production of oil is substantially greater than with water injection alone.

EXAMPLE III

An injection well is completed in the hydrocarbon-bearing formation in afield where the primary production has already been exhausted andperforations are formed between the interval of 4560-4580 feet. Aproduction well is drilled approximately 415 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 4565-4585 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

Water saturated with carbon dioxide at 1050 psig at a temperature of 70°F. and containing dissolved therein 0.28 weight percent of partiallyhydrolyzed polyacrylamide having about 18 mole percent of thecarboxamide groups originally present in the polyacrylamide hydrolyzedto carboxyl groups and having a number average molecular weight of about140,000 alkoxylated with about 14 weight percent of ethylene oxide inthe manner previously described in Example I above and containingdissolved therein 0.21 weight percent of a water-soluble, N-substitutedpolyacrylamide having a number average molecular weight of about 180,000bearing N-substituents of the formula:

    --(CH.sub.2 CH.sub.2 O).sub.2 CH.sub.2 CH.sub.2 SO.sub.3 Na,

is injected via the injection well into the formation at a pressure ofabout 1050 psig and at the rate of 1.1 barrels per minute. Injection ofthe driving fluid is continued at the rate of about 1.1 barrels perminute and at the end of 83 days the rate of production of oil issubstantially greater than with water injection alone.

EXAMPLE IV

An injection well is completed in the hydrocarbon-bearing formation in afield where the primary production has already been exhausted andperforations are formed between the interval of 5240-5260 feet. Aproduction well is drilled approximately 420 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 5245-5265 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

Water saturated with natural gas at 1200 psig at a temperature of 75° F.and containing dissolved therein 0.50 weight percent of partiallyhydrolyzed polyacrylamide having about 20 mole percent of thecarboxamide groups originally present in the polyacrylamide hydrolyzedto carboxyl groups and having a number average molecular weight of about2,500,000 alkoxylated with about 10 weight percent of ethylene oxide inthe manner previously described in Example I above and containingdissolved therein 0.25 weight percent of a water-soluble, N-substitutedpolyacrylamide having a number average molecular weight of about 180,000and bearing N-substituents of the formula:

    --(CH.sub.2 CH.sub.2 O).sub.4 CH.sub.2 CH.sub.2 SO.sub.3 H

is injected via the injection well into the formation at a pressure ofabout 1200 psig and at the rate of 0.95 barrels per minute. Injection ofthe driving fluid is continued at the rate of about 0.95 barrels perminute and at the end of 78 days the rate of production of oil issubstantially greater than with water injection alone.

I claim:
 1. A process for recovering hydrocarbons from a subterraneanhydrocarbon-bearing formation penetrated by an injection well and aproduction well which comprises:(A) injection into the formation via aninjection well a drive fluid comprising water having dissolved thereinabout 0.01 to about 5.0 weight percent of an N-substitutedpolyacrylamide bearing N-substituents of the formula:

    --(CH.sub.2 CH.sub.2 O).sub.m-1 CH.sub.2 CH.sub.2 SO.sub.3 M,

wherein m is an integer of from 2 to 5 and M is selected from the groupconsisting of hydrogen, sodium, potassium and ammonium, wherein thenumber average molecular weight of the N-substituted polyacrylamide isabout 10,000 to about 8,000,000, (B) forcing the fluid through theformation, and (C) recovering hydrocarbons through the production well.2. The process of claim 1 wherein the said drive fluid is saturated witha material selected from the group consisting of carbon dioxide,nitrogen, natural gas and mixtures thereof at a pressure of about 300 toabout 3000 psig.
 3. The process of claim 1 wherein about 0.005 to about0.3 weight percent of an alkaline agent selected from the groupconsisting of sodium hydroxide, potassium hydroxide and sodiumhypochlorite is included.
 4. The process of claim 1 wherein about 0.001to about 1.0 weight percent of a surfactant selected from the groupconsisting of alkylaryl sulfonates and alkyl polyethoxylated sulfates isincluded.
 5. A fluid comprising water having dissolved therein about0.01 to about 5.0 weight percent of an N-substituted polyacrylamidebearing N-substituents of the formula:

    --(CH.sub.2 CH.sub.2 O).sub.m-1 CH.sub.2 CH.sub.2 SO.sub.3 M,

wherein m is an integer of from 2 to 5 and M is selected from the groupconsisting of hydrogen, sodium, potassium and ammonium and wherein thenumber average molecular weight of the N-substituted polyacrylamide isabout 10,000 to about 8,000,000.
 6. The fluid of claim 5 saturated witha material selected from the group consisting of carbon dioxide,nitrogen, natural gas and mixtures thereof at a pressure of about 300 toabout 3000 psig.
 7. The fluid of claim 5 wherein about 0.005 to about0.3 weight percent of an alkaline agent selected from the groupconsisting of sodium hydroxide, potassium hydroxide and sodiumhypochlorite is included.
 8. The fluid of claim 5 wherein about 0.001 toabout 1.0 weight percent of a surfactant selected from the groupconsisting of alkylaryl sulfonates and alkyl polyethoxylated sulfates isincluded.